1. Field of the Invention
The present invention relates to a measuring apparatus for an optical disc, and particularly relates to a birefringence measuring apparatus for an optical disc substrate.
2. Description of the Prior Art
Some kind of resin, such as polycarbonate, is mainly used for a transparent substrate of an optical disc. An optical anisotropy, namely, birefringence of a molded transparent substrate is caused by an injection molding process because a direction and a speed of flow of a melted resin are fluctuated when the resin is injected from a center of a mold of the optical disc to a periphery thereof. Thus, the birefringence of the transparent substrate of the optical disc fluctuates along a radial direction of the optical disc, and does not fluctuate along a circumferential direction thereof.
Accordingly, it is required to select only a transparent disc having small birefringence which is allowable for an optical disc by evaluating the birefringence thereof after molding. A read only memory (ROM) type optical disc has a series of pits as a spiral information track formed thereon, and a reflective layer and a protective layer are formed successively thereon. A random access memory (RAM) type optical disc has a spiral groove thereon, and a memory layer, a reflective layer, and a protective layer are formed successively thereon.
Birefringence measuring methods are categorized into a transmission method and a reflection method.
The transmission method detects the birefringence of the transparent substrate having no reflective layer by using a laser beam which irradiates the transparent substrate vertically, and by detecting a transmitted light. The transmission method can measure precisely the birefringence of single transparent substrate having no reflective layer, but it cannot measure that of an adhered optical discs having a reflective layer.
The reflection method can measure the birefringence of an optical disc having a reflective layer. In this method, the laser beam penetrates the transparent substrate slantingly and is reflected by the reflective layer. An analyzer is revolved around an axis of a reflected laser beam, and a photodetector measures an intensity of the reflected laser beam. Then the birefringence of the optical disc is computed by utilizing a measured value of the polarization of the reflected laser beam.
In a prior art, a laser for emitting the laser beam is a HeNe gas laser or a single-mode laser diode, both of which have the laser beams of large coherence length.
In the reflection method, there are two reflections, one is a reflection from a surface of the transparent substrate, another is a reflection from the reflective layer of the optical disc. These two reflections easily interfere with each other, as the laser beam of the laser which is used in the prior art has a large coherence length. A photodetector detects these interfered reflections, which are changed in their intensities periodically by interference.
The interference is determined by amplitude and phase differences between the two reflections. The amplitude of the reflection from the surface of the transparent substrate is introduced by utilizing a refractive index of the transparent substrate, and that of the reflection from the reflective layer of the optical disc is introduced by using refractive indices of the reflective layer and of the transparent substrate. The difference of phase between the two reflections is determined by the thickness of the transparent substrate.
A wavelength of the laser beam used in this reflection method is in the range of several hundreds in nm. When the thickness accuracy of the transparent substrate is made in the order of nm over the whole area thereof, the interference caused by the two reflections can be estimated precisely in theory. But, actually, it is very difficult to know the thickness of the transparent substrate precisely in the accuracy of nm where the measurement is performed. Moreover, the thickness of the transparent substrate is about 0.6 mm for example, and irregularity of the thickness thereof is generally larger than several .mu.m, which is much larger than the wavelength of the laser beam. Thus, a value of the interference of the two reflections may change depending on the measured portion thereof, and cannot be estimated precisely. Accordingly, the birefringence measuring method of the prior art, which is forced to use interfered reflections, has a difficulty of removing the variable amplitude associated to the interfered reflections caused by the two reflections from the detected light beams. Therefore, the birefringence of the transparent substrate cannot be measured accurately.
As mentioned before, the ideal measurement of the birefringence is to irradiate the transparent substrate vertically. But, this measurement cannot be applied to the adhered optical disc, because beam axes of a light source and a reflected light are common to each other, and the light source and the photodetector cannot be positioned at the same axis. In this arrangement for measuring the birefringence, it is obliged to increase the incident angle of the laser beam from the vertical axis, so as to keep away the axes of the laser beam from the reflected beam.
In the birefringence measurement of the reflection method mentioned above, two reflections of which one is from the surface of the transparent substrate and another is from the reflective layer of the optical disc, interfere with each other. To avoid the interference of the two reflections, a beam radius of the laser has to be small, or the angle of the axis of the laser beam with respect to the disc surface has to be large. When the laser beam is formed as a parallel ray, the beam radius thereof is generally a few mm, and it is difficult to make it smaller. The birefringence strongly depends on the angle of the axis of the laser beam with respect to the disc surface, therefore, it is difficult to estimate the birefringence of vertical incidence from that of large angle. Therefore, it is difficult to measure accurately the birefringence of the transparent substrate of the optical disc by the birefringence measurement of the reflection method of the prior art.